I spend a lot of time in a discord server that allows musicians to play instruments and take requests for a bunch of listeners.

I used to play live with my grand piano with a studio mic but I've 'upgraded' to a MIDI keyboard and Pianoteq STAGE -

I route my MIDI keyboard through Pianoteq and then set the output of Pianoteq to VoiceMod (a recognisable audio input for discord) and then route this to Discord

It works fine and it seems to be very good quality for the listeners - the only issue is: because Pianoteq only allows for only one audio output at a time, it means I can't hear myself play whilst it's routed to Discord.

I was wondering if something exists that I could use to route from the output of pianoteq to two audio inputs (i.e. Voicemod and my headphones) so I could hear myself play?

Can’t you go into a DAW (which is hosting pianoteq as a plugin) first and then route the output into your server software as audio? A DAW has multiple outputs. How do you know what you’re playing currently if you can’t hear it back, maybe you’re a very good sight reader and it’s not a problem to you?